An organic EL device of the simplest structure is generally constituted of a light-emitting layer sandwiched between a pair of counter electrodes and utilizes the following light-emitting phenomenon. Upon application of voltage to the electrodes, electrons are injected from a cathode and holes are injected from an anode and they recombine in the light-emitting layer; after recombination, the energy level in the conduction band goes back to the energy level in the valence band with release of energy in the form of light.
In recent years, organic thin films have been used in the development of EL devices. In particular, devices that comprise a hole-transporting layer of an aromatic amine and a light-emitting layer of 8-hydroxyquinoline aluminum complex (hereinafter referred to as Alq3) disposed in thin film between the electrodes have been developed following the optimization of the kind of electrodes for the purpose of improving the efficiency of carrier injection from the electrodes and enhancing the luminous efficiency. The devices of this kind have produced remarkable improvement in luminous efficiency over the conventional devices utilizing single crystals of anthracene and the like and the developmental works of organic EL devices thereafter have aimed at commercial application to high-performance flat panels featuring self luminescence and high-speed response.
The utilization of phosphorescence in place of fluorescence is experimented to enhance the luminous efficiency of the device. The aforementioned devices comprising a hole-transporting layer of an aromatic diamine and a light-emitting layer of Alq3 and many others utilize fluorescence. Now, the utilization of phosphorescence, that is, emission of light from the triplet excited state, is expected to enhance the luminous efficiency approximately three times that of the conventional devices utilizing fluorescence (singlet). To achieve this object, studies were conducted on the use of coumarin derivatives and benzophenone derivatives in the light-emitting layer, but the result was nothing but extremely low luminance. Thereafter, europium complexes were tried in the utilization of the triplet excited state, but they failed to emit light at high efficiency. A large number of inventions have been made relating to phosphorescent dopants as cited in JP2003-515897 A (patent document 1).    Patent document 1: JP2003-515897 A    Patent document 2: JP2001-313178 A    Patent document 3: JP2002-305083 A    Patent document 4: JP2002-352957 A    Patent document 5: JP11-162650 A    Patent document 6: JP11-176578 A
In the development of organic EL devices, CBP that is a carbazole compound is proposed as a host material for use in the light-emitting layer as cited in JP2001-313178 A. However, the use of CBP as a host material for tris(2-phenylpyridine)iridium complex (hereinafter referred to as Ir(ppy)3) that is a phosphorescent material emitting green light destroys the balanced injection of electrical charges as CBP has a property of facilitating the flow of hole and obstructing the flow of electrons and excess holes flow out to the side of the electron-transporting layer to lower the luminous efficiency from Ir(ppy)3.
As a means to solve the aforementioned problem, a hole-blocking layer may be disposed between the light-emitting layer and the electron-transporting layer. The hole-blocking layer accumulates holes efficiently in the light-emitting layer thereby improving the probability of recombination of holes and electrons in the light-emitting layer and enhancing the luminous efficiency. Examples of the materials currently in general use for the hole-blocking layer include 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (hereinafter referred to as BCP) and p-phenylphenolato-bis(2-methyl-8-quinolinolato-N1,O8)aluminum (hereinafter referred to as BAlq). A hole-blocking material such as this can prevent electrons and holes from recombining in the electron-transporting layer. However, BCP lacks reliability as a hole-blocking material as it tends to crystallize easily at room temperature and a device comprising BCP shows an extremely short operating life. On the other hand, BAlq has a Tg of approximately 100° C. and a device comprising BAlq is reported to show a relatively long operating life, but the hole-blocking ability of BAlq is not sufficient and the luminous efficiency from Ir(ppy)3 drops.
On the other hand, 3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4-triazole (hereinafter referred to as TAZ) is also proposed as a host material for a phosphorescent organic EL device as cited in JP2002-352957 A; however, TAZ has a property of facilitating the flow of electrons and obstructing the flow of holes and the light-emitting range is displaced toward the side of the hole-transporting layer. Hence, it is conceivable that the luminous efficiency from Ir(ppy)3 may fall depending upon the compatibility of the material used for the hole-transporting layer with Ir(ppy)3. For example, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (hereinafter referred to as NPB) that is a material most widely used in the hole-transporting layer for its excellent performance, high reliability, and long operating life shows poor compatibility with Ir(ppy)3 and energy transition occurs from Ir(ppy)3 to NPB to lower the luminous efficiency.
The patent documents JP11-162650 A and JP11-176578 A disclose indolocarbazole compounds, but they disclose none of the compounds of this invention. Further, the disclosed indolocarbazole compounds are recommended for use as a host-transporting material and their stability is highly regarded, but the documents do not teach the use as a phosphorescent host material.